It does block the UI thread while it scans the directory and subdirectories selected looking for games to add.

I am not sure what the issue is here, since adding games tends to be an infrequent activity and to add single games, users normally select the specific game directory so the minor "pause" in UI interactivity should not be a problem.

If you are on macOS, you might be encountering a known random issue that causes the application to freeze for some users after adding a game (for example reported here and here. There are two ways that I know of to bypass the issue:

Use ScummVM 1.8.0 or 1.8.1 to add the games (and then you can play with ScummVM 2.0). It indeed seems that the issue only occurs with ScummVM 1.9.0 or more recent for some reason. This option is however only possible for games that were already supported in version 1.8.